It’s in bp-default/_inc/css/default.css where body has font-size of 12px.
To change, recommend you create a child theme https://codex.buddypress.org/theme-development/building-a-buddypress-child-theme/ and make the change there
You’ll need to be a little more specific about your set up and how you’re attempting this. What themes approach are you taking, are you using a custom theme, child theme of bp-default.
In theory you should be able to simply adjust the body font-size but a series of fixed px units throughout the layout make this harder to accomplish so you may have to target specific elements within the activities page.
The best and proper approach if you are using the bp-default theme is to create a simple child theme in which you simply have a style.css and in that you keep any custom styles.
Guides on creating a child theme: https://codex.buddypress.org/theme-development/building-a-buddypress-child-theme/
You’ll then need to identify the element you want to increase the size on. Depending on what elements you do actually want to increase you might simply be able to adjust the body font-size which is set to 12px but that will only change those elements that haven’t been explicitly set to a fixed size.
